The family of MutS homologs appear to collectively function as adenosine nucleotide regulated molecular switches which signal the timing of DNA and cellular processes such as mismatch repair as well as apoptosis in the event of overwhelming DNA damage. The heterodimeric MutS homologs hMSH4-hMSH5 appear to function uniquely in meiotic cells. Moreover, the transition from mitotic spermatogonia to primary spermatocytes in Meiosis I reveal a transition from the expression of hMSH2 to the expression of hMSH5. It is the goal of this research proposal to examine the function of hMSH4-hMSH5 using biochemical, molecular and genetic methodologies. They propose to: 1) characterize the functions and functional domains of hMSH4 and hMSH5 as well as their heterodimeric form hMSH4-hMSH5, 2) determine the functional interaction(s) between hMSH4-hMSH5 and the heterodimeric human MutL homologs, 3) determine the functional interaction(s) between hMSH4-hMSH5 and several human RecA homologs and recombination associated proteins, 4) biochemical characterization of the hMSH4-hMSH5 protein; including DNA binding and ATPase activities, and 5) delineation of the DNA structure(s) that provokes ADP-ATP exchange by hMSH4-hMSH5.